Fuser mechanism having crowned rolls

Abstract: A roll fuser assembly (33) including a fuser roll (34) and a pressure roll (36). The rolls (34,36) are crowned and are supported in pressure engagement with each other to form a fusing nip (65). The pressure engagement of the rolls eliminate nonuniform nip loading in wide fusers as well as providing uniform velocity through the fuser roll/pressure roll nip (65). It is preferred that the crowned profile of one of the rolls (32,34) be substantially to the center of the roll and that the other be offset therefrom. It is further preferred that the diameters of each of the rolls (32,34) be substantially the same at each end. A liquid release agent delivery or management system (71) includes a housing (73) containing oil, which is applied to the surface of the fuser roll (34) by a web (77) drawn from supply (76) to take up roll (78).

Description: The present invention relates to fuser apparatus for electrostatographic printing machines and in particular to oversized (i.e. wide rolls) roll fusers. In imaging systems commonly used today, a toner image is eventually transferred to a support surface, such as plain paper to which it may be permanently affixed by heating or by the application of pressure or a combination of both. In order to fix or fuse the toner material onto a support member or substrate, such as a sheet, permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with a material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon®. In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retrained by the heated fuser roller and not penetrate into the substrate surface The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate. In order to prevent this from happening, a release agent application mechanism is generally utilized. Wide, small diameter roll fusers also inherently suffer from excessive fuser and pressure roll deflection. The load on the fuser rolls required is a function of speed and type of image to be fused. Bending of a beam, or roller, is inversely proportional to the cube of the length thus, as fuser get wider the rolls bend appreciably more at a given load. Likewise, the bending of a beam with a round cross section, or roller, is directly proportional to the cube of the roll radius. So if it is desired to make the roll a little smaller the deflection increases significantly. The goal in a fuser nip is to produce nearly uniform load across the width. As the roll deflects the load at the ends increase thereby producing paper handing problems, if the load is too nonuniform (e.g. wrinkling or creasing of the sheet). It is known that skewing the fuser roll with respect to the pressure roll and wrapping one roll around the other tend to counteract the uneven load distribution caused by roll bending. However, the bent shape roll is a curve which is a cubic function, and it is being wrapped around a circular roll with is a squared function. Thus, resulting load distribution is a maximum about one quarter of the roll length in from each end to get a "bow tie" nip. Skewing has been successfully employed for fairly stiff systems and very flexible systems. The former needs very little compensation and thus little "bow tie" effect is apparent while the latter requires a lot of skew but the stiffness is low enough that the "bow tie"effect is not visible. Skewing also generates lateral thrust forces that wear the roll surface. It is also known to profile fuser rollers so that the shapes of the fuser tends to overcome the bending problem or even to place a third roll in pressure engagement with the fuser opposite the pressure roll to overcome the deflection in the ends of the fuser roll. Uneven roll load distribution can also be prevented by crowning one of the two fuser rolls. However, crowning of one of two fuser rolls results in nip velocity problems which induce paper wrinkle. US-A-4,727,392 discloses methods and apparatus for pressure developing imaging sheets in which rolls are journaled to form a nip for rupturing microcapsules on the surface of an imaging sheet as it is passed through the nip. In a two roll embodiment, the first and second rolls are crowned and pressed into contact with one another to present a uniform nip. US-A-4,200,389 discloses a pressure fixing apparatus in which copying paper with unfixed toner images formed on one surface is passed between a pair of press rollers in pressing contact with each other to fix the toner images to the copying paper with the pressure of the rollers. In accordance with one aspect of the invention there is provided an apparatus for fusing toner images to substrates, said apparatus comprising: first and second elongated rolls having crowned surfaces, said first and second rolls being supported for pressure engagement with each other to form a nip therebetween adapted to receive substrates, characterised in that the first roll comprises a pressure roll and the second roll comprises a fuser roll, and in that one of said first and second rolls has a maximum diameter in the central region thereof, and the other of said first and second rolls has a maximum diameter at an axial position offset from that of the maximum diameter of said one roll. According to another aspect of the invention, there is provided a method for fusing toner images to substrates, according to claim 7 of the appended claims. Other aspects of the present invention will become apparent as the following description proceeds and upon reference to the drawings, in which: Figure 1 is a enlarged elevational view of a fuser assembly incorporating the features of the present invention therein; and Figure 2 is an enlarged elevational view of the fuser roll and the pressure roll of Figure 1 shown in a non-engaged manner to illustrate the structure of the rolls. The present invention is suitably incorporated in an electrophotographic printing machine, the structure of which is well known in the art. A suitable machine is described (with reference to Fig. 3) in USSN 08/000,343, a copy of which was filed with the present application. The invention will now be discussed in detail with respect to Figure 1, in which a heat and pressure fuser apparatus 33 including a web release agent delivery system therefor, is schematically illustrated. As shown in Figure 1, the fuser apparatus 33 comprises a heated fuser roll 34 which is composed of a core 54 having coated thereon of a thin layer 56 of an elastomer. The core 54 may be made of various metals such as iron, aluminum, nickel, stainless steel, etc., and various synthetic resins. Aluminum is preferred as the material for the core 54, although this is not critical. The core 54 is hollow and a heating element 58 is generally positioned inside the hollow core to supply the heat for the fusing operation. Heating elements suitable for this purpose are known in the art and may comprise a quartz heater made of a quartz envelope having a tungsten resistance heating element disposed internally thereof. The method of providing the necessary heat is not critical to the present invention, and the fuser member can be heated by internal means, external means or a combination of both. Heating means are well known in the art for providing sufficient heat to fuse the toner to the support. The thin fusing elastomer layer may be made of any of the well known materials, for example, RTV and HTV silicone elastomers. The fuser roll 34 is shown in a pressure contact arrangement with a pressure roll 36. The pressure roll 36 comprises a metal core 62 with a layer 64 of a heat-resistant material. In this assembly, both the fuser roll 34 and the pressure roll 36 are mounted on bearings (not shown). The pressure roll bearings are mechanically loaded,as schematically indicated by the arrow 63 so that the fuser roll 34 and pressure roll 36 are pressed against each other under sufficient pressure to form a nip 65 It is in this nip that the fusing or fixing action takes place with toner images contacting the heated fuser roll 34. The layer 64 may be made of any of the well known materials such as fluorinated ethylene propylene copolymer or silicone rubber. The liquid release agent delivery or management system 71 of the present invention comprises a housing 73 containing release agent material, for example, silicone oil. The silicone oil is applied to the surface of the fuser roll 34 via a web of material 77 which is impregnated with the oil which is drawn from supply 76 to a take up roll 78. The web material 77 is impregnated with silicone oil and upon contact with the fuser roll 34, it delivers silicone oil thereto. The web material 77 contacts the fuser roll at a nip formed between the fuser roll 34 and a pinch roll 79 formed of an open cell material for applying a thin coating of silicone thereon for preventing offset of images carried by a paper substrate. The liquid release agent may be selected from those materials which have been conventionally used. Typical release agents include a variety of conventionally used silicone oils including both functional and non-functional oils. Thus, the release agent is selected to be compatible with the rest of the system. It is preferred that the release agent delivery system 71 be of the type disclosed and discussed in U.S. Patent Application Serial No. 08/000,151. A copy of the description thereof was filed with the present application; compare US-A-5 327 203. Various other systems have been used to deliver release agent fluid to the fuser roll including the use of oil soaked rolls and wicks with and without supply sumps as well as oil impregnated webs. Another type of RAM system is disclosed in US-A 4,214,549. As disclosed therein, release agent material is contained in a sump from which it is dispensed using a metering roll and a donor roll, the former of which contacts the release agent material and the latter of which contacts the surface of the heated fuser roll. The pressure roll 36 is crowned as is the fuser roll 34, as best seen in the enlarged elevational view of Figure 2. The pressure roll 36 is supported in pressure engagement with the fuser roll 34 as indicated by arrow 63 in Figure 1. The pressure roll 36 is crowned or larger in the center region of the roll than the ends to compensate for the bending of the fuser and pressure rolls, whereas the fuser roll 34 is crowned at an offset region to again account for the bending. The amount of radial increase (crown) is the sum of the deflection of the fuser roll and pressure roll combination. The fuser roll stiffness is relatively unimportant so it can be made relatively thin and light so it warms up fast and is relatively inexpensive in power usage and construction. In a two roll fuser, warm-up time is an outgrowth of the roll mass required for adequate stiffness. In this embodiment, a sheet 82 (Figure 1) with toner 81 thereon is advanced to and through the nip 65 to affix images carried by the sheet to the sheet. In this case, the image has been transferred to the sheet 82 at a transfer station D. The transfer station D includes a corona generating device 32 proximate a moving photoconductive surface 12 between which the sheet has been directed to transfer the toner thereto, as is common in electrophotographic printers. The sheet, after passing through the nip 65, is directed toward an output tray 38 via reversing roll set 37 and associated guides or baffled surfaces. Stripper means 87 such as are well known in the art may be used to ensure separation of the sheet 82 from the fuser roll surface 56 after passing out of the nip. Finally, as will be also understood, the control signals for the sheet handler operation are provided by controller 100 (see Fig. 3 of USSN 08/000,343), which is preferably a conventional microprocessor system, as is well known. It is contemplated that the controller controls all machine steps and functions described herein, as well as that of any and/or all apparatus and devices associated with the sheet handler, such as, for example, an electrophotographic printing machine. By way of example, the fuser and pressure roll lengths are in the order of 10 to 36 inches (25.4 to 91.4cm) In an embodiment reduced to practice, the fuser and pressure outer surfaces were 12 inches long (320 mm). Also, by way of example the fuser roll wall thickness is approximately 0 2 inches (5.5mm) and has a diameter of approximately 1.3 inches (32.2mm) thereby providing a fairly low mass fuser roll capable of rapid warmup. The light weight fuser roll is about 10-20% as stiff as the steel pressure roll which has approximately 1.14 inches (28.5mm) diameter and a wall thickness of approximately 0.5 inches (12.7mm). The increase in diameter of the fuser roll from and to maximum diameter, for the roll of the embodiment reduced to practice should increase from end to maximum width in a range between approximately 0.004 and 0.060 inches (0.01 and 0.15 mm), and preferably, approximately 0.020 inches (.05 mm) (i.e. , in the preferred embodiment reduce to practice the surface of the fuser roll varies in diameter from approximately 1.3 inches to 1.32 inches (32.2 to 32.25 mm ). The diameter of the pressure roll increases from the ends to a maximum diameter in a range from between approximately 0.020 to 0.400 inches (0.05 to 1.0 mm) from end to maximum diameter with approximately 0.144 inches (0.36 mm) preferred (i.e., in the preferred embodiment reduce to practice the surface of the pressure roll varies in diameter from approximately 1.14 to 1.297 inches (28.5 mm to 28.86 mm)). This arrangement yields a substantially uniform sheet velocity profile for sheets passing through the nip. The velocity profile for such sheets is such that it increases only approximately 0.5% from the center of the rolls to edges of the rolls Further, the arrangement also provides a relatively uniform nip pressure. It will be appreciated by those skilled in the art that the increase in velocity at the edges of sheets to be fused is actually a desirable condition. That is, it is enough of an increase to prevent creasing and yet not so much as to induce buckling of sheets, generally. In this embodiment, the offset of the increase in diameter of the fuser roll to the increase in diameter of the pressure roll compensates for the fact that sheets are intended to pass along a registration edge when entering the nip. It will be understood by those skilled in the art that in a fuser assembly where the sheets are centrally fed relative to the rolls the maximum diameter of both rolls would be in the central region. In this embodiment, with a fuser roll and pressure roll, each approximately 12 inches (320 mm) long with a registration edge positioned approximately 0.5 inches (12.7mm) from one edge, it is preferred that the pressure roll be crowned at its center (e.g., approximately 5.8 inches (149mm) from the registration edge along which sheets are fed to the fuser apparatus) and that the fuser roll is crowned at a distance approximately between approximately 6.75 and 9 inches (180 and 240mm) with the preferred distance being approximately 6.3 inches (160mm) from the registration edge. With the previously referenced sizes, the nip which is formed varies in size from approximately 0.175 inches (4.44 mm) proximate the registration edge to about 0.1650 inches (4.19mm) at the center to about .1800 inches (4.57mm) proximate the far edge (in this embodiment approximately 279.5 mm, 11 in.). The normal force or sheets in the nip varies from about 21.5 lbs./in. (3.76 N/mm) proximate the registration edge to about 17.25 lbs./in. (3.02 N/mm) at the center to about .23 lbs./in. (4.03 n/mm) proximate the far edge. In one embodiment of the present invention it has been found desirable to employ a fuser roll having a maximum diameter positioned approximately one third of the length thereof The fuser roll engages a pressure roll having a maximum diameter at the center thereof. This permits fusing sheets of various sizes without skewing or wrinkling the sheets. In this arrangement, relatively narrow sheet (e.g., A-5 short-edge feed) and larger sheet (e.g., A-4 short-edge feed) are fed along the registration edge. In recapitulation, in the present invention the fuser roll and pressure roll are crowned in a barrel or convex shape. One of the rolls, either a pressure roll or a heated fuser roll (preferably the pressure roll), is crowned or larger in the central region than the ends and the other is crowned along its length but is larger at a region disposed away from the central region. By adjusting the shapes accordingly, compensation for the bending of the rolls is provided. The amount of radial increase for each roll (crown) is a function of the deflection of the particular roll so that the fuser roll/pressure roll combination provides relatively equal pressure across the length of the fuser/pressure roll up. In this manner, the stiffness of the pressure roll and fuser roll is relatively unimportant so each can be made thin and light allowing for greater cost and performance flexibility. This is particularly true in a two roll fuser as warm-up time is an outgrowth of the roll mass required for adequate stiffness, and cost is generally an outgrowth of roll mass. Further, the load (i.e., the normal force) on sheets in the nip is substantially uniform along the length of the rolls in the nips. The tangential velocity of the rolls, across their length, is different since the rolls vary in diameter along their lengths. This results in the surface speed varying due to the different radial dimensions of the rolls. The fuser roll and pressure roll nip, however, provides a relatively constant speed profile for sheets because the deflection strain is uniform and the effective circumference formed by the rolls in the nip region is substantially uniform The speed profile of a sheet in the nip, thus, is substantially constant across the sheet. A slight sheet speed profile increase from center to the edges of approximately .5% is contemplated and beneficial. The total size and weight of a fuser is usually an important consideration in design, and the lesser weight and smaller size with increased performance provided hereby are useful.

Claims: Apparatus for fusing toner images (81) to substrates (82), said apparatus comprising: first and second elongated rolls (34, 36) having crowned surfaces, said first and second rolls (34, 36) being supported for pressure engagement with each other to form a nip (65) therebetween adapted to receive substrates (82), characterised in that the first roll comprises a pressure roll (36) and the second roll comprises a fuser roll (34), and in that one of said first and second rolls (34, 36) has a maximum diameter in the central region thereof, and the other of said first and second rolls has a maximum diameter at an axial position offset from that of the maximum diameter of said one roll. Apparatus according to claim 1, further comprising means (63) for applying substantially uniform pressure and velocity on substrates (82) in the nip (65). Apparatus according to claim 2, wherein said applying means (63) comprises loading means for deflecting said pressure roll (36) into pressure engagement with said fuser roll (34) to form said nip (65). Apparatus according to claim 1, 2 or 3, further comprising means for supporting said pressure roll rotatably in contact with said fuser roll. Apparatus according to claim 3, wherein the crowned surface of said fuser roll and pressure roll defines a profile approximately corresponding to the sum of the deflection of said fuser roll and said pressure roll. Apparatus according to any one of the preceding claims, wherein said fuser roll (34) comprises a surface having a maximum diameter axially positioned approximately one third of the length from one end thereof. A method for fusing toner images to substrates, comprising the steps of: providing first and second elongated rolls having crowned surfaces; and supporting the first and second rolls in pressure engagement with each other to form a nip therebetween adapted to receive substrates, characterised in that the first roll comprises a pressure roll (36) and the second roll comprises a fuser roll (34), and in that one of said first and second rolls (34, 36) has a maximum diameter in the central region thereof, and the other of said first and second rolls has a maximum diameter at an axial position offset from that of the maximum diameter of said one roll. The method according to claim 7, further comprising the steps of applying uniform pressure in the nip and effecting substantially uniform velocity on substrates in the nip.

Inventors: DEBOLT FREDERICK C, PARISI RUSSELL A, RASCH KENNETH R, DEBOLT, FREDERICK C., PARISI, RUSSELL A., RASCH, KENNETH R.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1994
Main classification: G03G15
IPC codes: F16C13, G03G15